How can the sewage water get mixed with the fresh water on a modern 1 year old ship?

On December 20, 2022 my wife and I were returning from Antarctica on relatively large ship (holds nearly 200 passengers).

Our cabin was on the 6th level.

While going across the Drake passage (between Antarctica and the southern tip of Argentina) the ship went through 6 to 8 meter waves and brown water came out from our the shower, sink and also the toilet when flushed.

We stopped touching any water from our bathroom after that.

How can this happen on a new one year old cruise ship that was designed specifically to maneuver the high seas of the Drake passage? Just curious.

Any plumbing experts out there?